Drying apparatus and method



Feb. 7, 1967 D. GRAHAM DRYING APPARATUS AND METHOD 2 Sheets-Shet 1 Filed Sept. 9, 1964 A AAAAM MMM D. L. GRAHAM Feb. 7, 1967 DRYING APPARATUS AND METHOD 2 Sheets-Sheet 2 Filed Sept. 9, 1964 r INVENTOR. flag/a5 1.622295),

BY Y

W 9 fldZvM/mys.

United States Pater 6 3,302,297 DRYWG APPARATUS AND METHGD Douglas L. Graham, Wingate, Ind. (REA, Gibson City, Ill. 60936) Filed Sept. 9, 1964, Ser. No. 395,249 Claims. (Cl. 34-13) This invention relates generally to methods and apparatus for drying particulate material, and more particularly to a method and apparatus for drying grain, such as corn.

The drying of grain involves a mass-heat transfer process in which three basic methods have heretofore been employed; the cross-flow system, the counter-flow system and the concurrent flow system. Grain dryers of both the portable and stationary variety, and of both the batch type and continuous-flow type have been provided employing each of these drying systems.

In the cross-flow system of grain drying, the drying air flows through a stationary bed of grain in case of the batch-type apparatus or flows through a moving bed of grain in a direction perpendicular to the direction of grain flow in the ease of continuous-flow type of apparatus. A characteristic of the cross-flow system is the development of a temperature and moisture gradient across the bed of grain with the result that the inner layers of grain approach the initial drying-air temperature before the outer layers are dry.

In the counter-flow drying system, the drying air flows through a moving bed of grain in a direction opposite to the direction of grain flow. In this system, the driest air is placed in contact with the driest grain and thus the drying rate will be comparatively constant throughout the bed. However, the final grain temperature in the counter-flow system will be about the initial temperature of the drying air and the exhaust air temperature will be about the wet bulb temperature of the grain prior to drying. In the counter-flow system, therefore, the maximum temperature of the drying air is restricted since the maximum temperature of the grain should not exceed about 140 F. since higher temperatures will greatly lower the germination rate, Furthermore, if the bed is too deep, the last layers of grain may be moistened thus lowering the air temperature below the wet bulb temperature.

In the concurrent-flow system of grain drying, the air flow and the grain flow are in the same direction with the result that the grain is subjected to very high drying rates at the start of the drying process with the drying rate rapidly declining as the air becomes saturated. I have found that as the drying rate is increased for low moisture corn, the quality of the dried corn is reduced, whereas, within limits, high moisture corn can withstand high drying rates with little change in quality, thus indicating that the concurrent-flow drying process is most desirable. However, it is also desirable rapidly to cool the corn after drying and prior to delivery.

Here, it has been observed that high cooling rates tend to cause stress cracks and it is therefore desirable that a low and constant cooling rate be employed. As indicated above in connection with the drying process, the counterflow system providesa comparatively constant rate of heat transfer thus indicating that the counter-flow system will provide the lowest and most constant cooling rate.

ice

It is accordingly an object of my invention to provide improved drying apparatus.

Another object of my invention is to provide an improvide grain drying method.

A further object of my invention is to provide an improved drying apparatus and method obtaining the advantages of concurrent-flow drying and counter-flow coolmg.

In accordance with my invention, I have provided a drying apparatus and method employing concurrent-flow drying and counter-flow cooling with both the drying air and cooling air being exhausted together at a point intermediate the moist and cool-dry ends of the bed, respective y.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a side elevational view of a portable continuous-fiow grain dryer incorporating my invention;

FIG. 2 is an end view of the dryer of FIG. 1;

FIG. 3 is a fragmentary cross-sectional View taken along the line 33 of FIG. 2; and

FIG. 4 is a fragmentary cross-sectional view taken along the line 44 of FIG. 3.

Referring now to the figures of the drawing, the improved grain dryer of the invention, generally identified at 10, comprises an air-tight upstanding container 12, preferably fabricated from suitable sheet metal, and having top and bottom ends 14, 16, side walls 18, 20 and end walls 22, 24. The top and bottom ends 14, 16 are preferably V-shaped to take advantage of the angle of repose of grain in distributing the grain entering the container 12 and collecting the grain leaving the container as will hereinafter be more fully described.

In the illustrated embodiment, container 12 together with the other apparatus to be hereinafter described, is mounted upon a trailer frame 26 which in turn, is supported by a pair ground engaging wheels 28, 30. The apparatus is adapted to be towed by means of a suitable trailer hitch 32, as is well known to those skilled in the art.

Grain to be dried is deposited in a suitable hopper 34 and then conveyed by conventional augers 36, 38 disposed externally of the conveyor to a point adjacent the apex 40 of the top end 14 of the container 12. The grain conveyed upwardly by auger 38 is then deposited upon the entrance auger 42 which extends between the end walls 22 24 of the container 12 adjacent apex 40 of the top end 14. A trough element 44 cooperates with the portions of top end 14 adjacent apex 40 thereof to enclose the auger 42 thereby to permit the grain to be conveyed axially by the auger from end wall 24 toward end wall 22, the trough element 44 having a plurality of apertures 46 formed therein for admitting the grain conveyed by the auger 42 to the interior of the container 12. A pair of baffle plates 48 may be provided depending from the trough element 44 for directing the grain discharged by the auger 42 to the interior of the container 12. A pair A partition 52 extends horizontally across container 12 between side walls 18, 20 and end walls 22, 24 adjacent but spaced from the bottom end 16 to define cavity 54.

Partition 52 is formed by a plurality of inverted V-shaped elements 56 and has suitably grain metering apertures 58 formed in the bottom of the troughs 6t) defined between the V-shaped elements 56. Suitable metering rolls 62 are positioned in the troughs 6t) and cooperate with the apertures 53 to withdraw dried grain from tie interior of container 12 to the cavity 54. As will be hereinafter more fully decribed, the portions of the V-shaped elements 56 between the troughs so are perforated, as at 64, to admit cooling air to the interior of the container 12.

The cooled and dried grain is removed from the con tainer 12 by a suitable auger 66 extending between the end walls 22 24 adjacent the apex 58 of the bottom end 16. Auger 66 transfers the dried and cooled grain to an external elevating auger 68 having a conventional discharge spout 70 at its upper end (FIG. 2) for delivering the dried and cooled grain, as desired.

In the initial operation of the apparatus thus far described and illustrated, the entrance auger 4-2 is operated prior to operation of the metering rolls 62 and discharge auger 65 so as to accumulate a bed 72 of grain in the container 12 with the bottom end of the bed being supported upon the partition 52 and the bed filling container to a level such as that shown by the dashed line 74, i.e., so that the bed 72 has a substantial depth between its top and bottom ends. That part of the container 12 which reecives and contains the bed 72 is referred to as the bed-retaining portion, the upper end portion thereof being in the vicinity of the dashed line 74 and the lower end portion thereof being in the vicinity of the partition 52. Once the bed '72 of grain has been accumulated, the metering rolls 62 are rotated, as by means of suitable external sprockets 76 and a drive chain 78 as seen in FIG. 2, thereby to withdraw grain from the bottom end of the bed 72 while grain is being added to the top end '74 by the entrance auger 42 with the result that the grain in bed 72 moves or flows downwardly in the container 12 as shown by arrow 86. The discharge auger 66 is also operated to remove the grain transferred from bed 72 to the cavity $4 by the metering rolls 62. The drying and cooling operation is simultaneously initiated, as will be described hereinafter, it being understood, of course, that immediately following initial operation of the device as above-described, the grain discharged by conveyors 66, 68 will have to be returned to the dryer via the hopper 34 and conveyors as, 38, 452 until the grain being discharged has been found to be sufiiciently dry.

In order to provide for the concurrent-flow of heated drying air, a plurality of ducts 82 are provided at one location extending axially between end walls 22, 24 in transversely spaced apart parallel relationship, ducts 82 being vertically spaced above partition 52 so as to be adjacent the top end 14 of container 12, as shown. These ducts 82 may be formed of sheet metal to a shape wherein the side walls are straight and substantially vertical and the upper wall is of inverted V-shape. The lower wall 84 is of upright V-shape and perforated to provide for the free flow downwardly of air from the duct 82. The angle of the lower wall 84 is made slightly greater for example) than the angle of repose of the grain to cause the latter to maintain contact with the downwardly converging surfaces of the wall so as to prevent the development of a gap or void directly beneath the duct $2 which could collect dust and similar matter. Such dust remaining in such void could create a fire hazard after drying. Enough perforations are provided in the wall 84 to render the underside of the duct 32 substantially open.

In situations where temperatures of fire-hazard proportions are not reached, it is not necessary to use the perforated V-wall 34 such that the ducts 32 are completely open on the undersides thereof.

In the operation of the apparatus, the level of repose or top end 74 of the bed 72 of grain should be above the lower perforated ends 84 of the ducts 82.

End wall 22 is provided with a plurality of openings 86 respectively communicating with the ducts 82, as best seen in FIG. 3. A chamber 88, also preferably fabricated from suitable sheet metal, is mounted on the exterior side of end wall 22 and communicates with the openings 86 and with the cavity 54-. A conventional blower 90 is provided in end wall 92 of the chamber 88 for creating a static pressure therein for moving air through the ducts $2 and the cavity 54, bafile members 94, 96 being provided more efiiciently to direct the flow of air from the fan 9t? to the ducts 82 and cavity 54- as shown by the arrows 93, 190. A suitable heater 102 is provided in the portion 164 of the chamber 88 defined by baffle 94 for heating the air entering the ducts 82.

It will now be seen that the heated air which enters the ducts S2 flows axially over the grain in the bed 72 and then downwardly in the bed, as shown by the arrows 106, in the same direction as the direction of movement iii of the grain in the bed. This is concurrent flow. It will be seen further that the unheated air which enters the cavity 54 initially flows axially in the cavity and then enters the bottom of the bed through the perforations 64 of the partition 52 flowing upwardly through the bed as shown by the arrows 16 8; it will be seen that the direction of flow 108 of the cooling air is opposite from the direction of flow 80 of the grian in the bed 72. This is counter-flow.

Both the concurrent-flow of heated drying air and the counter-flow of cooling air are exhausted from the bed 72 and the container 12 by a plurality of inverted V-shaped ducts Ht) extending horizontally across the container 12 between the side walls 18, 2t), i.e., transversely with respect to the ducts 82 and the V-shaped elements 56 of the partition 52. The exhaust ducts 110 are vertically spaced between the entrance ducts 82 at one location and the partition 52 at another location and are open at their bottoms 112, as shown. The side walls 18, 2d of container 12 have a plurality of openings 114, 116 formed therein respectively communicating with the opposite ends of the ducts Ht). It will now be seen that the downwardly flowing heated drying air 1G6 will enter the open lower ends of the exhaust ducts 110 as shown by the arrows 118, and that the upwardly flowing cooling air 10% will also enter the exhaust ducts 110, as shown by the arrows 120, both the heated, drying air and the cooling air thus being exhausted together by the ducts 110 from the bed '72 and the container 12 in a direction transverse to the directions of air flow 106, 108 and the direction of grain flow 80, as shown by the arrows 122. The space between the entrance ducts 82 and exhaust ducts 114) may be regarded as a heating air-flow zone whereas the space between inlet partition 52 and ducts 1th may be regarded as a cooling air-flow zone. The terms air-flow or gas-flow zone are herein used to mean the structure disclosed and the equivalents thereof as well as any portion of the bed through which air or gas is caused to how. It is inherent that in the portion of the bed occupied by the heating air-flow zone, an air temperature gradient is developed which decreases from a maximum at ducts 82 toward the ducts 110, and in the portion occupied by the cooling air-flow zone, an air temperature gradient is developed which increases from said partition 52 toward said ducts 116. These gradients are established during a normal, continuous grain-drying operation of this invention.

Blower 20 is chosen so as to maintain sufficient air pressure to force the heated drying air to flow downwardly through bed 72 to and into the exhaust ducts 110 and so as to force the cOOling air upwardly through bed 72 to and into the exhaust ducts 110. It will further be seen that metering rolls 62 serve to move the grain in bed '72 down through the container 12 in layers so that every kernel of grain receives the same drying and cooling treatment.

In a specific embodiment of the invention, container 12 is dimensioned to "accommodate a bed 72 of grain from five to six feet in depth, the heater 102 is a low pressure gas burner providing 3,000,000 Btu. per hour and the blower 90 provides 9,000 c.f.m. at 3.5 inches of water, this air flow being divided by bafiles 9'4, 96 to provide 6,000 c.f.m. to the heater portion 104 of chamber 88 and 3,000 c.f.m. to the cooling cavity 54. With this arrangement in which the container 12 was six feet wide by fourteen feet long to provide a bed area of eighty-four square feet, initial drying air temperature of 350 F. was provided and a final air and grain temperature at the exhaust ducts 110 of 115 F. with the grain flow rate of 2.04 bushels per hour per square foot; grain was dried from 23% wet basis to a final moisture content of 13% wet basis. In this specific embodiment, six (6) entrance ducts 82 are provided respectively ten inches high, six inches wide and spaced apart by six inches, the ducts 82 being positioned four and one-half (4 /2) feet above the apices of the inverted V-shaped elements 56 of partition 52. Fourteen (14) exhaust ducts 110 were provided respectively ten inches high, six inches wide and spaced apart six inches, ducts 110 being spaced above the 'apices of elements 56 of partition 52 by one and one-half (1 /2) feet.

It will be readily understood that while the apparatus and method of the invention have been described in connection with the driving of grain, particularly corn, it is equally suitable with appropriate modifications in temperatures and air flow rates for drying other particulate materials. Furthermore, While air has been described as the drying and cooling medium, it will be understood that other gaseous media may be employed.

By the expression, as used in certain of the claims hereafter, undried, unheated, wet grain is meant grain that has not immediately prior to encountering the concurrent flow of heated air from ducts 82, been subjected to any preheating or predrying treatment.

While there have been described above the principles of this invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention.

What is claimed is:

1. The method of drying grain comprising the steps of: forming a bed of grain of substantial thickness between opposite ends; moving said grain in a direction from one end toward the other; adding undried, unheated, wet grain to be dried at one of said ends and withdrawing dried grain at the other of said ends; providing a first concurrent flow of a gaseous medium through a first portion of said bed in said given direction from a point adjacent said one end toward said other end; providing a second counter fiow of gaseous medium through a second portion of said bed in the opposite direction from a point adjacent said other end toward said one end; the temperature of the concurrent flow medium being higher than the temperature of the counter flow medium; the second portion of said bed being the next succeeding por tion of said bed which carries a flow of gaseous medium following said first portion; there being only one each of said concurrent and counter flow portions within said bed; and exhausting both of said flows from said bed at a point intermediate said ends.

2. The method of drying grain comprising the steps: forming a bed of grain of substantial depth between top and bottom ends; continuously adding undried, unheated, wet grain to be dried on said top end and continuously withdrawing dried grain from said bottom end so that the grain in said bed continuously moves downwardly; directing a first concurrent flow of heated drying air downwardly through a portion of said bed in a direction from said top end thereof; directing a second counter fiow of cooling air upwardly through a portion of said bed in a direction from said bottom end thereof; there being only one each of said concurrent and counter flow of air within said bed; and exhausting both of said flows from said bed at a point generally midway between said top and bottom ends, said concurrent and counter-flow portions being sequentially arranged, said counter-flow portion being the next portion of said bed through which air is directed following said concurrent flow portion.

3. The method of drying grain comprising the steps of forming a bed of grain to be dried, said bed having opposite end portions, passing in one direction grain to be dried through said bed from one end portion to the other, the grain at said one end portion being undried, unheated and wet, said bed being divided into only two sequentially arranged air-flow zones, one zone extending from one end portion of said bed toward the other end portion, the other zone extending from the other end portion of said bed toward said one end portion, providing a flow of heated air through said one zone only in said one direction, providing a flow of cooler air in said other zone only in the opposite direction, said other zone being the next succeeding air-flow zone following said one zone when progressing in said one direction, exhausting said air from a location between said zones, whereby an air temperature gradient in said bed is produced which decreases from said one end portion toward said exhausting location and furthermore increases from said other end portion toward said exhausting location.

4. The method of drying grain comprising the steps of forming a bed of grain to be dried, said bed having opposite end portions, passing in one direction grain to be dried through said bed from one end portion to the other, introducing undried, unheated, wet grain into said bed at said one end portion and extracting drier grain from the other end portion, subjecting said undried, unheated, wet grain in said bed to a concurrent flow of heated drying air adjacent to said one end portion for raising the temperature and imparting a drying action to said grain, subjecting the driest grain in said bed to a counterfiow of cooler air adjacent to said other end portion for reducing the temperature of the heated grain, exhausting the air of both the concurrent and counterfiows from said bed at a location intermediate said end portions.

5. Drying apparatus comprising: a material drying container for accommodating a bed of particulate material of substantial depth; means for introducing material to be dried into said container on top of said bed; means for withdrawing dried material from said container from the bottom of said bed whereby said material moves downwardly in said container between introduction and withdrawal thereof; first means for conveying a first flow of a heated gaseous medium horizontally in a first direction across said bed adjacent the top thereof, said first means including means for'introducing said first flow to said bed for downward movement therethrough throughoutsubstantially the entire cross-sectional area thereof; second means for conveying a second flow of a cooler gaseous medium horizontally in said first direction across the bottom of said bed, said second means including means for introducing said second flow to said bed through the bottom thereof for upward movement therethrough throughout substantially the entire cross-sectional area thereof; and third means intermediate said first and second means extending horizontally for exhausting both of said flows from a region in said bed intermediate said first and second means, the heated gaseous medium thereby fiowing concurrently with the direction of material movement and the cooler medium flowing in a direction counter thereto, means including said first means and said third means for limiting the flow of said heated gaseous medium to said concurrent direction only; means including said second means and said third means for limiting the flow of said cooler gaseous medium to said counterdirection only.

6. Drying apparatus comprising: an upstanding material drying container having top and bottom ends; partition means extending horizontally across said container for supporting a bed of material; means adjacent said top end of said container for continuously introducing particulate material to be dried thereto, said partition means being spaced from said introducing means whereby a bed of material of substantial depth may be accumulated; means for continuously removing dried material from said container, said partition means having openings therein for withdrawing said dried material from the bottom of said bed whereby said material in said bed moves downwardly between introduction and withdrawal thereof; first duct means extending horizontally across said container and spaced between said partition means and top end, said first duct means having downwardly facing openings therein for introducing a first concurrent flow of a gaseous medium to said bed adjacent the top thereof for downward movement through said bed; said partition means having other openings for introducing a second counter flow of gaseous medium to the bottom of said bed for upward movement through said bed; second duct means extending horizontally across said container and spaced between said first duct means and said partition means, said second duct means communicating with a side wall of said container and having openings for exhausting both of said flows from said bed; and means for introducing said gaseous medium under pressure to said first duct means and to said partition means thereby providing said first and second flows therein.

7. Drying apparatus comprising a material-drying container having top and bottom portions, said container also having a bed-retaining portion between said top and bottom portions for accommodating a bed of particulate material to be dried, said bed-retaining portion having upper and lower end portions, means for introducing gaseous media at said upper and lower end portions of said bed-retaining portion, means for exhausting said gaseous media from said bed-retaining portion in a region between said upper and lower end portions whereby flow of said gaseous media is vertically opposite from said upper and lower end portions into said exhausting means, means including said introducing means and said exhausting means for limiting the flow of said gaseous media only to the vertically opposite directions aforesaid, said introducing means including means for maintaining the temperature of the gaseous media introduced at the upper end portion higher than that introduced at the lower end portion; the region between said lower end portion and said exhausting means next succeeding the region between said upper end portion and said exhausting means, means for retaining a bed of particulate material in said bedretaining portion, and means for causing vertically downward movement of the particulate material in said bed including means for introducing said material into the top portion of said container and withdrawing material from the bottom portion thereof, whereby the portion of the material moving downwardly between said upper end portion and said exhausting means is accompanied by the concurrent flow of said gaseous media while that portion moving downwardly between said exhausting means and said lower end portion is accompanied by the counterflow of said gaseous media.

8. Drying apparatus including means for retaining a bed of particulate material, means for moving the material in said bed in a predetermined direction, said bed having opposite ends and a depth extending parallel to said direction, means for introducing gaseous media at two locations into said bed, said locations being adjacent to said opposite ends, respectively, means for exhausting said gaseous media from within said bed in a region between said two locations whereby flow of said gaseous media occurs from said two locations into said exhausting means, a first gas-flow Zone being defined by and disposed between one location and said exhausting means in which said gaseous media flows in said predetermined direction, a second gas-flow ZOne being defined by and disposed between the other location and said exhausting means in which said gaseous media flows in a direction opposite to said predetermined direction, there being only the aforesaid first and second gas-flow zones within said bed, the gaseous media introduced at said one location being of a higher temperature than that introduced at said other location thereby establishing a first gas temperature gradient in said first zone which progressively decreases from a position adjacent to said first location to said exhausting means and a second gas temperature gradient in said second zone which progressively increases from said other location toward said exhausting means, said first and second zones being arranged in succession with said second zone being the next succeeding gas-flow zone following said first zone, said one location being the first of the two locations encountered in moving in said predetermined direction whereby the wettest particulate material moving in said bed receives thereon the highest temperature gaseous media.

9. Drying apparatus comprising: an upstanding material drying container having top and bottom ends and opposite side and end walls; a partition extending horizontally across said container between said side and end walls for supporting a bed of material, said partition being spaced from said bottom end and defining a cavity therewith; means adjacent said top end of said container for continuously introducing particulate material to be dried thereto thereby to form and add material to said bed from the top, said partition being vertically spaced below said introducing means whereby a bed of material of substantial depth may be accumulated; said partition having a plurality of material metering openings formed therein communicating with said cavity; material metering means cooperating with said metering openings for continuously removing dried material from the bottom of said bec to said cavity; means in said cavity for continuously removing dried material from said container; a first plurality of transversely spaced parallel ducts extending horizontally between said end walls, said first ducts being vertically spaced between said partition and said introducing means, said first ducts having downwardly facing openings therein for introducing a first concurrent flow of a heating gaseous medium to said bed adjacent the top thereof for downward movement through said bed, one of said end walls having openings formed therein respectively communicating with said first ducts; said partition having other openings therein intermediate said metering openings and communicating with said cavity for introducing a second counter flow of a cooling gaseous medium to the bottom of said bed for upward movement through said bed; a second plurality of transversely spaced parallel ducts extending horizontally between said side walls, said second ducts being vertically spaced between said partition and said first ducts, said side walls having openings formed therein respectively communicating with said second ducts, said second ducts having downwardly facing openings for exhausting both of said flows from said bed and said container in a direction trasverse to said downward and upward movement; a chamber on said one end wall and communicating with said opening therein and said cavity for furnishing said gaseous medium thereto; means in said chamber for heating said first flow; and means for creating a static pressure in said chamber thereby providing said first and second flows.

10. Drying apparatus comprising a material drying container for accommodating a bed of particulate material of substantial depth, said container having top and bottom end portions, means for introducing material to be dried into the top end portion of said container for forming said bed, means for withdrawing material from the bottom end portion whereby said material moves downwardly in said container between introduction and Withdrawal thereof, means for introducing heated gaseous medium into an upper portion of said container, means 9 for introducing cooler gaseous medium into a lower portion of said container, means for exhausting said gaseous media from said container at a location intermediate and spaced from said upper and. lower container portions thereby producing a first downward flow of gaseous medium from said upper portion to said location and a second upward flow from said lower portion to said location, means including the first-mentioned introducing means and said exhausting means for limiting the flow of said heated gaseous medium within said container to a generally downward direction only, and means including the second-mentioned introducing means and said eX- hausting means for limiting the ilow of said cooler gaseous medium within said container to a generally upward direction only.

References Cited by the Examiner UNITED STATES PATENTS 1,100,397 6/1914 Reynolds 34-168 1,239,216 9/1917 Randolph 34-168 2,635,864 4/1953 Goins 263-19 10 MARTIN P. SCHWADRON, Primary Examiner.

FREDERICK L. MATTESON, IR., Examiner.

B. L. ADAMS, Assistant Examiner. 

3. THE METHOD OF DRYING GRAIN COMPRISING THE STEPS OF FORMING A BED OF GRAIN TO BE DRIED, SAID BED HAVING OPPOSITE END PORTIONS, PASSING IN ONE DIRECTION GRAIN TO BE DRIED THROUGH SAID BED FROM ONE END PORTION TO THE OTHER, THE GRAIN AT SAID ONE END PORTION BEING UNDRIED, UNHEATED AND WET, SAID BED BEING DIVIDED INTO ONLY TWO SEQUENTIALLY ARRANGED AIR-FLOW ZONES, ONE ZONE EXTENDING FROM ONE END PORTION OF SAID BED TOWARD THE OTHER END PORTION, THE OTHER ZONE EXTENDING FROM THE OTHER END PORTION OF SAID BED TOWARD SAID ONE END PORTION, PROVIDING A FLOW OF HEATED AIR THROUGH SAID ONE ZONE ONLY IN SAID ONE DIRECTION, PROVIDING A FLOW OF COOLER AIR IN SAID OTHER ZONE ONLY IN THE OPPOSITE DIRECTION, SAID OTHER ZONE BEING THE NEXT SUCCEEDING AIR-FLOW ZONE FOLLOWING SAID ONE ZONE WHEN PROGRESSING IN SAID ONE DIRECTION, EXHAUSTING SAID AIR FROM A LOCATION BETWEEN SAID ZONES, WHEREBY AN AIR TEMPERATURE GRADIENT IN SAID BED IS PRODUCED WHICH DECREASES FROM SAID ONE END PORTION TOWARD SAID EXHAUSTING LOCATION AND FURTHERMORE INCREASES FROM SAID OTHER END PORTION TOWARD SAID EXHAUSTING LOCATION. 